The present invention relates to ablation apparatus and methods and to steerable devices, including those used in cardiac ablation
Contraction or “beating” of the heart is controlled by electrical impulses generated at nodes within the heart and transmitted along conductive pathways extending within the wall of the heart. Certain diseases of the heart known as cardiac arrhythmias involve abnormal generation or conduction of the electrical impulses. One such arrhythmia is atrial fibrillation or “AF.” Certain cardiac arrhythmias can be treated by deliberately damaging the tissue along a path crossing a route of abnormal conduction, either by surgically cutting the tissue or applying energy or chemicals to the tissue, so as to form scar. The scar blocks the abnormal conduction. For example, in treatment of AF it has been proposed to ablate tissue in a partial or complete loop around a pulmonary vein within the vein itself near the ostium of the vein; within the ostium; or within the wall of the heart surrounding the ostium. It would be desirable to perform such ablation using a catheter-based device which can be advanced into the heart through the patient's circulatory system.
As described in co-pending, commonly assigned U.S. Patent Application Ser. No. 09/905,227, published as US/2002/0065512-A1 (the “'512 publication”) and granted as U.S. Pat. No. 6,635,054, the disclosures of which are hereby incorporated by reference herein, an expansible structure is used as a reflector for directing and focusing ultrasonic waves from an ultrasonic transducer into a region of tissue to be ablated. As further described in the '512 publication, certain preferred embodiments according to that disclosure include an expansible structure incorporating a structural balloon which is inflated with a liquid and a reflector balloon inflated with a gas. The balloons share a common wall. The balloons are configured so that the common wall has a generally parabolic shape. Because the liquid in the structural balloon and the gas in the reflector balloon have substantially different acoustic impedances, the interface between the balloons at the common wall is a nearly perfect reflector for ultrasonic waves. Ultrasonic waves are emitted from a small transducer within the structural balloon and passes radially outwardly from the emitter to the reflector. The reflector redirects the ultrasonic waves and focuses it into a ring-like ablation region encircling the central axis of the emitter and balloons. This ablation region is just forward of the structural balloon. Thus, the ultrasonic waves will ablate tissue in a region encircling the central axis or forward-to-rearward axis of the balloon structure.
This arrangement can be used, for example, to treat atrial fibrillation by ablating a circular region of myocardial tissue encircling the ostium of a pulmonary vein. The ablated tissue forms a barrier to abnormal electrical impulses which can be transmitted along the pulmonary veins and, thus, isolates the myocardial tissue of the atrium from the abnormal impulses. To provide effective treatment in this mode of operation, the ring-like focal region should encircle the ostium and should lie in a plane which is parallel or nearly parallel with the inner surface of the heart. In some embodiments disclosed in the '512 publication, the structural balloon is provided with a forwardly projecting tip at its central or forward-to-rearward axis, so that by engaging the tip in the lumen of the pulmonary vein, the forward-to-rearward axis of the balloon structure can be placed at the center of the ostium. A guide wire can be threaded into the pulmonary vein. The balloon is then advanced along the guide wire until the tip lodges in the ostium in the pulmonary vein. Where the particular pulmonary vein being treated has a main trunk which extends generally perpendicular to the interior surface of the heart wall, and where the ostium has the expected configuration, this arrangement works properly.
However, there is significant variability in the anatomy of the pulmonary veins and their ostia. For example, that portion of the pulmonary vein adjacent the ostium may lie at an oblique angle to the interior surface of the heart wall. In order to engage the tip of the structural balloon in such an ostium, the forward-to-rearward axis of the balloon must be tilted at a comparable angle, so that the ablation region is unintentionally tilted relative to the interior surface of the heart wall. Also, two or more pulmonary veins may join one another close to a common opening or ostium or may be enlarged or shaped so that it is difficult to engage the tip in the ostium. Moreover, even where the patient has the desired, nominal anatomy, it has been difficult to confirm proper placement of the balloon assembly. Thus, still further improvements would be desirable.
The delicate tissues within the pulmonary vein can be damaged by forcibly engaging structures with these tissues and by moving the engaged structures while the structures are forcibly engaged with the tissues. It would be desirable to provide an improved system and method which does not rely on such forcible engagement to orient the balloon or other ablation device in the desired disposition. Further, it is often necessary or desirable to move an ablation device to several different dispositions within the heart chamber. For example, the treatment plan may require formation of loop-like lesions around the individual ostium of each of several pulmonary veins. It would be desirable to provide apparatus and methods which facilitate such repositioning.
Further, it has been proposed that more effective treatment can be provided by ablated generally linear lesions along the heart wall in conjunction with loop-like lesions. However, heretofore it has been proposed to form the linear lesions using specialized devices as, for example, catheters equipped with a point energy source such as a single pair of electrodes for applying RF energy, so that the linear lesion can be traced by moving the catheter so as to move the single point source along the heart wall or, alternatively, by catheters equipped with numerous energy emitters such as numerous RF electrodes disposed along the length of the catheter. Such a catheter may be provided as a separate device which must be separately introduced into the heart, thus complicating and prolonging the procedure. Alternatively, it has been proposed to provide such a catheter as a portion of a catheter carrying a device for forming a loop-like lesion. Although this approach theoretically simplifies the task of positioning the needed devices within the heart, in fact, it substantially complicates the construction of the device and also complicates the tasks of positioning each individual device. Thus, further improvement in this regard would also be desirable.
Moreover, further improvements in construction of expansible ablation devices, including balloon-based ablation devices, would be helpful. In particular, it would be desirable to provide improved structures which facilitate cooling of a piezoelectric ultrasonic emitter, and structures which can reinforce the expansible device when the same is in an expanded condition. It would also be desirable to provide a back-up system which would minimize the consequences in the unlikely event of a structural failure in one or more components of the device. It would be desirable to provide these improvements without substantially increasing the diameter of the expansible ablation device when the same is in a collapsed condition.